1. Special Field of the Invention
The invention relates to high-fidelity loudspeaker units designed more specially for undistorted reproduction of bass tones and whose three-dimensional reproduction is better than that of prior art loudspeaker cabinets and the space in which sound reproduction takes place seeming to be greater than its true size.
2. The Prior Art
In normal loudspeaker unit design, all single loudspeakers are fixed to a common support (or acoustic) wall for radiation forward. For reducing the shortcomings of a unidirectional characteristic in the case of high pitch or treble radiation, however, a number of different treble or tweeter loudspeakers have been placed with an angular spacing between them, i.e., angularly offset, so that their directional diagrams are overlapping over the greatest possible angle for producing the evenest possible acoustic radiation pressure. An extreme case of such a system is the so-called spherical loudspeaker unit in which a great number of single high-pitch or treble systems are placed on a generally spherical face, so that a generally regular omnidirectional radiation effect is produced. The structure of such a system is, however, very complex.
Furthermore, even before the development of true sterophonic reproduction, "3D sound" was publicized in advertisements, in which, in addition to the front loudspeaker, smaller loudspeakers were placed at the two side walls of a wireless receiver housing for radiation of the medium and higher frequency ranges. The purpose of this was to produce a more omnidirectional radiation of the medium and higher frequencies, so that the medium and higher frequency acoustic performances are not only sensed well in the front direction, but also to the side of the receiver.
For bass emphasis so-called bass reflex units or cabinets have been designed (see, for example, the Austrian Pat. No. 212,897), in which a low pitch or bass loudspeaker is placed on one wall of a chamber for radiation to the outside through an acoustic opening in this chamber wall, while the backside of the diaphragm of the loudspeaker is utilized for oscillation of the air within the chamber. The chamber has, in addition to the acoustic opening, a second opening, referred to as the reflex opening, which is connected with a duct having an air column in it, and which, in order to make the air column longer, may be bent a number of times. The duct is generally formed by an inner structure within the loudspeaker unit. Together with the air volume in the chamber and the diaphragm of the loudspeaker, this air column takes the form of an oscillating means whose resonant frequency is lower than that of shut or closed loudspeaker cabinets by the reciprocal of the square root of two, so that the overall cabinet or loudspeaker enclosure will be responsible for radiation even of low frequencies with a great displacement volume. However, in this case, the radiation face is in no way large enough in size. The size of a bass loudspeaker diaphragm for operation without distortion, must have the same size relation to the average backing wall behind the orchestra, as the musical instrument with the greatest face area to the backing wall of the auditorium, or, more specifically, as the average overall face area of the musical instruments used to the backing wall, this number being divided by the square root of the overall number of musical instruments being played. Diaphragms of this order of size will, however, be overly heavy and not stiff enough, contributing to further distortion. Furthermore, in this case, the volumes of the housings are excessively large.
As disclosed in the German specifications (Offenlegungsschriften) No. 2,111,581 and No. 2,116,962 and the Austrian specification No. 305,403, an attempt has been made to use inner housing parts for copying an exponential horn joined with a relatively small loudspeaker chamber and which is designed for emission of the sound, radiated from the back side of the diaphragm of the loudspeaker, while decreasing the radiation impedance at the back side of the diaphragm, over a large area and into the auditorium as well. These inner structures are, however, generally complex and high in price and, for this reason, have not been put into general use, particularly because the lower limiting frequency in such structures is generally high. In the case of a housing and horn, made smaller in size to be in line with more modern developments, the resonant frequency f.sub.o and the lower limiting frequency f.sub.G are even higher. In a third way of making a shorter horn, by way of compensation, the modulus of widening (m%) and the horn mouthpiece diameter d.sub.e are decreased once again for increasing the dynamic air weight. However, such a horn is responsible for producing a false pitch because standing waves are formed so that it has a "trumpeting effect".
Leaving this unsatisfactory line of development, the applicant herein first designed a loudspeaker unit referred to as a so-called "chamber horn," in which an exponential horn, decreased in size, is placed at the reflex opening of a chamber. The selection of the volume of the chamber together with the resonant frequency of the loudspeaker and the size of the connection opening (in this case the reflex opening has the effect of a low pass filter) between the chamber and the horn is designed to produce a desired low resonant frequency so that the exponential horn is responsible for substantially improving the radiation impedance matching (radiation face matching) of the chamber opening to the surrounding space in line with the impedance transformation properties of an exponential horn. Because of the chamber size noted earlier, the low base frequencies may be present at the connection opening to the horn with a relatively great oscillation amplitude and in-phase with the acoustic waves, radiated from the front side of the diaphragm, and the chamber has the effect of a pressure chamber as well because of this connection opening, the exponential horn for bass radiation may have very much smaller dimensions than would be necessary for a normal design of exponential horn loudspeaker for comparable bass radiation. So, using simple design measures, a good and powerful bass reproduction is made possible.